1. Field of the Invention
The present invention relates generally to a wheel support bearing assembly in the form of a hub-bearing assembly and, more particularly, to the wheel support bearing assembly having a relatively high load bearing capacity, which is suitably used in commercial vehicles such as trucks and station wagons and also in passenger cars in general.
2. Description of the Related Art
A vehicle wheel support bearing assembly for passenger cars, of a kind utilizing an angular contact ball bearing has developed from the first generation bearing, in which a seal-equipped dual row rolling bearing is used, to the second generation hub bearing, in which an outer race concurrently serves as a flanged wheel hub and, then, from the third generation bearing, in which inner and outer races include flanges, to the fourth generation bearing of a type integrated with a constant velocity universal joint.
The third generation wheel support bearing assembly includes, as shown in FIGS. 12 and 13, a hub axle 34 which forms a part of a rotatable inner member and which has an outboard raceway groove 33a defined on an outboard outer peripheral surface thereof, and a non-rotatable flanged outer member 32, and a separate inner race member 35 mounted on the inboard outer peripheral surface of the hub axle 34 and having an outer peripheral surface formed with an inboard raceway groove 33b. The wheel support bearing assembly shown in FIG. 12 is used for rotatably supporting a vehicle driven wheel, whereas the wheel support bearing assembly shown in FIG. 13 is used for rotatably supporting a vehicle drive wheel. In the wheel support bearing assembly of FIG. 12, a nut 40 is threadedly mounted on an externally threaded portion 39 so formed as to protrude from an inboard end of the hub axle 34 to fix the inner race member 35 immovable on the hub axle 34 in an axial direction of the wheel support bearing assembly.
The Japanese Laid-open Patent Publication No. 11-51064 discloses a wheel support bearing assembly utilizing a tapered roller bearing. To describe this realted art wheel support bearing assembly with the aid of FIG. 12 or 13, the patent publication suggests a hardened layer 50 formed on a portion of the outer peripheral surface of the hub axle 34 so as to extend axially between the inboard end thereof and a root 42a of a flange 42, which layer 50 is formed by means of a high frequency heating technique.
A portion of the hardened layer 50 adjacent the root 42a of the flange 42 serves to enhance the rigidity of the flange 42. Considering that, in the wheel support bearing assembly having a high load bearing capacity, such as used in trucks and station wagons, increase of the rigidity of the flange and its surrounding portion has been required, the use of the hardened layer 50 appears to effectively satisfy this requirement.
On the other hand, another portion of the hardened layer 50 where the inner race member 35 rests serves to increase the creep resistance of the interface between the hub axle 34 and the inner race member 35. While attempts have been made to apply a friction preventing agent to the interface between the inner race member 35 and the outer peripheral surface of the hub axle 34, this application is complicated and time-consuming and, therefore, it has been desired to increase the creep resistance by the utilization of the hardened layer 50.
In the third generation wheel support bearing assembly of an inner race rotating type, however, instead of the use of the bolt and nut for fixing the inner race member on the hub axle such as shown in FIG. 12, it is now a mainstream to use the structure in which the inboard end of the hub axle is staked to fix the inner race member on the hub axle.
Although the structure in which the inner race member 35 is fixed by staking appears to be excellent in terms of productivity and cost, cracking tends to occur in the hardened layer 50 if the inner end of the hub axle is staked while the hardened layer 50 on the outer peripheral surface portion of the hub axle 34 has been formed so as to extend to the inboard end of the hub axle 34. As a result, the inner race member 35 fails to be fixed in position. Also, if the hardened layer 50 extends only to a position away from the inboard end of the inner race member 34, it would be difficult to increase the strength of the hub axle 34 and also to secure the creep resistance at the interface between the inner race member 35 and the hub axle 34.